Videos from the Department of Elastomer friction and contact mechanics

High Speed linear test rig "HiLiTe"

Videos “Cavitation in Water with Barbell Sonotrode“ and “Cavitation in Water with Cylindrical Sonotrode“

Cavitation is the formation of small air or gas bubbles which pulsate and implode in liquids. This causes high local pressures and temperatures that can be used for various chemical reactions, but also for example in cleaning applications. When a liquid is subjected to ultrasonic excitation, the resulting phenomenon is called acoustic cavitation. The changes of pressure generated by the ultrasonic transducer increase the static pressure in the liquid. Rapid changes of pressure cause the formation of cavities that implode when subjected to higher pressure. In the video “Cavitation in Water with Barbell Sonotrode“, an ultrasonic transducer generates a very rapid change of pressure in water. The cavitation is generated by a dumbbell-shaped sonotrode attached to the ultrasonic transducer. The result is a high-speed cavitation jet reaching from the sonotrode’s tip to the bottom of the container. In the areas around the cavitation jet, there are larger bubbles or large bunches of bubbles that move in the liquid. To make the cavitation bubbles visible, the water container is illuminated from below.

In the video “Cavitation in Water with Cylindrical Sonotrode“, the cavitation is generated by a cylindrical sonotrode which is attached to the ultrasonic transducer. The resulting rapid change of pressure creates two main cavitation fields in the water. One cavitation field is created directly at the sonotrode’s tip, the second one near the container bottom. Flashing occurs from the bottom-near cavitation field towards the container bottom and towards the cavitation field at the sonotrode’s tip. To make the cavitation bubbles visible, the water container is illuminated from below.

Cavitation in Water with Barbell Sonotrode

Cavitation in Water - Cylinder

When placing a sound source at a certain distance parallel to a reflector, this can cause a standing wave, meaning that the points at which the amplitude of the wave is minimum or maximum or crosses zero are fixed in the sound field. The phenomenon can also be generated by ultrasound. The tip of an ultrasonic transducer serves as sound-emitting surface. When placing it at a certain distance parallel to a reflecting surface, this causes a standing wave. In case of standing waves in air, it is possible that small parts levitate in the sound field. The respective experiments were documented by means of a high-speed camera.

In the video “Standing Wave Levitation – Droplet Injection”, it can be observed how water is injected with a hollow needle into a standing wave field. The water drops are partly atomized due to the high pressure in the sound field. The resulting fine water droplets make the sound field visible. It can also be seen how the water droplets move in the sound field before leaving it and falling to the bottom. In the video, the aim was to inject a water droplet into the sound field and then manipulate it by means of acoustic excitation.

If water droplets can be positioned in the sound field, it is possible to hold the droplets in a state of levitation and make them also vibrate. This is shown in the video “Standing Wave Levitation – Droplet Manipulation”. When adding a low-frequency vibration to the high-frequency ultrasonic field, the droplets perform different movements. The strongest displacements can be obtained when excited at one of the so-called eigenfrequencies. At such frequencies, the droplets are easily excitable to characteristic mode shapes.